In olefin epoxidation, feedstocks containing an olefin and an oxygen source are contacted with a catalyst disposed within a reactor under epoxidation conditions which results in the production of olefin oxide and typically includes unreacted feedstock and combustion products. The catalyst usually comprises a catalytically active material, such as silver, deposited on a plurality of ceramic pellets which may be referred to as carrier. Processes for making carrier are described in U.S. Pat. No. 6,831,037 and U.S. Pat. No. 7,825,062.
The technology used to manufacture carriers that are desirable for use as catalyst supports in an olefin epoxidation reaction has evolved substantially over the last few decades. In U.S. Pat. No. 4,007,135 (Hayden), which issued on Feb. 8, 1977, the description of example 4 discloses a carrier sold by Norton Co. wherein the “porosity to water was 25%” and the surface area of the carrier was 0.36 m2/g. The description of example 7 in Hayden discloses a support which had a water porosity of 16 to 20% and a surface area of 0.17 m2/g. In contrast to the descriptions in examples 4 and 7 in the Hayden reference, which may be generally described as disclosing carriers having low surface area and low pore volume, U.S. Pat. No. 5,187,140 (Thorsteinson), which issued on Feb. 16, 1993, discloses “a high surface area, high porosity carrier” (see column 6, lines 32-33) for the epoxidation of alkene to alkylene oxide. In column 7, lines 40-51, Thorsteinson describes the carriers of the subject invention as having a surface area greater than about 0.7 m2/g and, preferably, having a water pore volume of at least about 0.55 cc/g and most preferably from about 0.6 to about 0.8 cc/g. The '140 reference also discusses the teachings of the EP 0,327,356 (Jin); and U.S. Pat. No. 4,829,043 (Boehning) in the Background of the Invention section of the specification. The Jin reference is described as disclosing a carrier having “a total pore volume greater than 0.5 milliliters per gram, preferable 0.5 to 0.7 milliliters per gram” and “a surface area of 0.2 to 2 m2/g, preferably 0.8 to 1.3 m2/g”. The Boehning reference is described as disclosing a carrier that “has a surface area of 0.4 to 0.8 m2/g and a pore volume of not less than 0.45 milliliter per gram.” While the information in these references generally indicates that the technology used to manufacture carriers for catalysts used in the production of alkylene oxides has evolved from dense (i.e. low pore volume) and low surface area carriers to porous (i.e. high pore volume) and high surface area carriers there have been a few disclosures of low pore volume, high surface area carriers. For example, the '140 reference identified above also discloses CARRIER “AC” which was described as “available from the Norton Company, Stow, Ohio as 5502” and had a surface area of 0.80 m2/g and water pore volume of 0.26-0.32 cc/g. In another reference, Example 1A in US 2009/0192324 discloses an alpha alumina carrier having the following characteristics “(specific surface area: 1.0 m2/g; water absorption: 35.7% by weight; SiO2 content: 3.0% by weight; Na2O content: 0.35% by weight;”. The general trend in the technical evolution of carriers described above, which has continued for approximately two decades, is believed to have occurred because the disclosed carriers did not provide the desired performance when used as a catalyst support.
A key driver behind the technical efforts to provide an improved catalyst has been to reduce the manufacturing cost of a reactor's final product (i.e. an olefin oxide) such as ethylene oxide. The cost of manufacturing can be impacted, both positively and negatively, in several ways which may be interrelated and thus complicated to isolate and improve upon. For example, the cost of the final product can be reduced if the selectivity of the reaction can be increased without a corresponding increase in the reactor's operating temperature. As used herein, selectivity is an indication of the proportion, usually represented by a percentage, of the converted material or product which is alkene oxide. If the carrier and catalyst can be changed so that the selectivity of the reactor is improved, then a higher percentage of the reactants are converted to the desired final product relative to the percentage of reactants converted with a previously used catalyst. The cost of the final product may also be reduced if the reactor's operating temperature can be reduced relative to another carrier that has generally equivalent or lower selectivity. Another tactic to reduce the cost of the final product is to improve the longevity of the catalyst which means that the reactor can be operated for longer periods of time before the selectivity and/or activity of the catalyst declines and/or the temperature increases to an unacceptable level which requires the reactor to be stopped so that the catalyst can be replaced. Stopping the reactor to replace the catalyst inherently incurs expenses that add to the cost of the final product.
With regard to the evolution of carrier and catalyst technology, the inventors of this application have recognized that there is a strong symbiotic relationship between changes made to the carrier and subsequent changes made to the catalyst which collectively improve or degrade the economic performance of the reactor. For example, as described above, some commercially available carriers have had low pore volumes, such as less than 0.35 g/g of catalyst, which may have limited the amount of catalytically active material (i.e. silver) which could be deposited. Limiting the amount of silver per gram of catalyst inherently limited the amount of silver per unit of volume within the reactor. However, carriers with total pore volume below 0.35 g/g, which may also be described as high density carriers, were resistant to crushing and abrasion which were desirable characteristics. Furthermore, the chemical composition of the carrier was substantially influenced by the impurities in the commercially available raw materials used to make the carrier. Some of the raw materials were the alumina, bond material and pore formers. Each of the raw materials had the potential to intentionally (or unintentionally) import excessive levels of certain compounds, such as Na2O, SiO2 and potassium containing compounds which could adversely impact the performance of the catalyst. To improve the performance of the catalyst researchers began to develop carriers that were more porous than their predecessors thereby increasing the amount of silver which could be deposited. Evidence of the move to developing more porous carriers can be found in the teachings in U.S. Pat. No. 7,547,795 (Matusz) which describes carriers of similar surface area, but with varying water absorption values. Furthermore, this patent teaches that increasing the water absorption of the carrier “allows for the loading of a greater amount of silver onto the support material than can be loaded onto other inorganic materials that have a lower water absorption.” As the amount of silver per gram of carrier is increased, the amount of silver per unit of volume within the reactor is also increased which leads to improved selectivity and longevity. Unfortunately, increasing the porosity of the carrier reduces the carrier's resistance to crushing and increases its abrasion which are both undesirable attributes.
The goal of producing a carrier and catalyst that is both resistant to crushing and abrasion and enables selectivities and longevities beyond those commercially available has heretofore been difficult to achieve because of the perceived conflict between making a carrier with good resistance to crushing and abrasion while also providing useful porosity to allow for enough silver to be deposited onto a carrier and subsequently loaded into a reactor. The inventors of the invention described and claimed below have recognized that a carrier having certain micro physical and chemical characteristics, as will be explained, can improve the selectivity of the catalyst while providing a physically robust carrier thereby reducing the cost of the desired final product.